The medial olivocochlear efferent system is almost universally present in mammalian auditory systems, yet the mechanisms by which it acts and its functional significance are still unclear. Our previous work suggests that efferent inhibition observed at low and high sound levels may have entirely different physiological bases, the former through the outer-hair- cell (OHC) synaptic conductance shunting OHC receptor currents, the latter through electrical effects on inner hair cells and afferent neurons. An analysis of the implications of these low and high sound level mechanisms, and our preliminary results, indicate that current views about the pattern of efferent effects with changes in sound frequency, sound level, efferent firing rate, and the number of efferents firing are fundamentally incorrect. The proposed experimental work will test specific hypotheses, formulated from our past work, about the mechanisms of efferent inhibition both at low sound levels and at high sound levels. The data obtained will generate a new and more complex picture of how discharge patterns in auditory nerve fibers are affected by various levels of efferent activity. These data will enable us to predict how sensory input from the ear is modified under many different conditions so that functional consequences can be deduced. This knowledge should aid in understanding both normal and pathological hearing and add to the growing understanding of feedback control systems in neurobiology.